This invention relates to automatic tier control of a storage system that has a plurality of storage medium tiers.
The popularization of solid state drives (SSDs), which are faster than hard disk drives (HDDs), has brought a diversification of storage media that are installed in a storage system, and efficient use of storage medium tiers, particularly high performance tiers, is now a matter to be solved. As a solution to this matter, an automatic storage tier placement function has become popular with which data is automatically stored in an appropriate storage medium based on the I/O load of the data. For instance, a monitoring system is disclosed in US 2012/0246386 A1, in which a load index value defined from the viewpoint of a long cycle and a load index value defined from the view point of a short cycle are updated based on the number of I/Os that is counted cyclically for each storage area. Promotion (moving a page to an upper tier) and demotion (moving a page to a lower tier) are executed based on a value that is calculated from the load index values.
Meanwhile, data compression technologies and data deduplication technologies are popular as solutions to the matter of an increased data amount. The I/O of compressed or deduplicated data usually requires data restoration processing, e.g., decompression, which causes a drop in I/O performance. Technologies that use a fast compression function in an SSD to reduce the amount of data while maintaining I/O performance have been proposed. For instance, a storage system that includes a plurality of flash packages and a storage controller configured to receive a write request from a host and to transmit data requested in the write request to one of the plurality of flash packages that is the write destination is disclosed in US 2012/0317333 A1. The storage controller defines, for each flash package, a virtual capacity larger than the physical capacity of the flash package. The storage system compresses write data, and writes the compressed write data to the write destination flash package.
A larger amount of heavy-load data can be placed in upper-tier storage media by using the tier placement function described above in combination with a fast data amount reducing function that does not affect I/O performance. The hit ratio of upper tiers in this case is higher than when the tier placement function is used alone, and an improvement in I/O performance is accordingly expected.